In situ measurement of seasonal δ¹⁸O variations and analysis of isotopic trends in a modern speleothem from southwest Australia

We present a record of seasonal and inter-annual oxygen (δ¹⁸O) and carbon (δ¹³C) isotope ratios from an 81-year-old stalagmite from Moondyne Cave, southwest Australia. The growth history of stalagmite MND-S1 is known since it grew on a cave boardwalk that was installed in 1911 and removed in 1992. This stalagmite provides an excellent test of speleothem climate proxies because the regional climate is strongly seasonal (wet winter/dry summer) and has experienced a 200 mm (20%) reduction of mean rainfall since the mid-1960s, and a 0.8 °C temperature rise since ∼ 1953. Seasonal variations in calcite δ¹⁸O were measured in situ by high spatial resolution ion microprobe, whilst inter-annual variations of δ¹⁸O and δ¹³C were measured by conventional gas-source mass spectrometry. Comparison of the speleothem stable isotopes and instrumental temperature records reveals that δ¹⁸O variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall δ¹⁸O measurements show that the mean seasonal range in δ¹⁸O of rainfall in southwest Australia is large (2‰) and inversely correlated with rainfall amount. A rainfall driver for the speleothem δ¹⁸O is confirmed by the detection of seasonal shifts of 0.7-1.5‰ in speleothem δ¹⁸ that track rainfall δ¹⁸O, smoothed by storage in the overlying limestone. The seasonal range in speleothem δ¹⁸O is larger than any inter-annual and decadal variation observed in the record. The prominent annual cycles in speleothem δ¹⁸O revealed by ion microprobe analysis indicate that subtle changes in the frequency of intense winter rainfall events, or possibly also moisture sources, could produce significant changes in mean speleothem δ¹⁸O. The ion microprobe results also raise the possibility that the masses of speleothem calcite deposited in winter and summer could vary as a function of the seasonal drip rate and carbonate saturation state of these waters. If this is the case, then small changes in the relative masses of calcite deposited in winter and summer could produce significant shifts in mean δ¹⁸O and δ¹³C that have a complex relation to climate. This finding should be generally applicable to the interpretation of long-term trends in speleothem geochemical records for shallow cave sites where seasonal variations in geochemical tracers are relatively large, including most of the sub-tropical monsoon belts and mid to high latitudes with distinct rainfall seasons.